The present invention is directed to golf balls utilizing improved polybutadiene compositions for use in molded golf ball cores in conjunction with a particular type of cover composition. In one aspect, the improved polybutadiene compositions utilize one or more particular butadiene rubbers synthesized through the use of neodymium and cobalt-containing catalysts. The polybutadiene is preferably an ultra-high Mooney viscosity polybutadiene. In another aspect, the improved polybutadiene compositions utilize a particular solid butadiene rubber that exhibits an ultra-high Mooney viscosity and/or a high molecular weight and a low dispersity. The use of such butadiene rubber and/or blend of butadiene rubbers increases the resiliency of the ball. In addition, significantly improved mixing properties are achieved. In another aspect, the golf balls feature particular cover constructions that result in the balls exhibiting a soft feel and particular mechanical impedance.
Two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (referred to as xe2x80x9cC.O.R.xe2x80x9d), also expressed as the constant xe2x80x9cexe2x80x9d, which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact, or more generally, the ratio of the outgoing velocity to incoming velocity of a rebounding ball. As a result, the coefficient of restitution (i.e. xe2x80x9cexe2x80x9d) can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision. Hardness is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball""s diameter. The lower the compression value, the harder the material.
Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determine the distance a ball will travel when hit. Since clubhead speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
In this regard, the coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and electronically measuring the ball""s incoming and outgoing velocity. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (xe2x80x9cU.S.G.A.xe2x80x9d). Along this line, the U.S.G.A. standards indicate that a xe2x80x9cregulationxe2x80x9d ball cannot have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball""s initial velocity (i.e. as the C.O.R. of a ball is increased, the ball""s initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. impact resistance) to produce enhanced durability.
The coefficient of restitution (C.O.R.) in solid core balls is a function of the composition of the molded core and of the cover. In balls containing a wound core (i.e., balls comprising a liquid or solid center, elastic windings, and a cover), the coefficient of restitution is a function of not only the composition of the center and cover, but also the composition and tension of the elastomeric windings.
Polybutadiene has been utilized in forming golf ball cores. Prior artisans have investigated utilizing various grades of polybutadiene in core compositions. For example, such attempts are described in U.S. Pat. Nos. 5,385,440; 4,931,376; 4,683,257; 4,955,613; and 4,984,803; and in Japanese Patent References JP 58225138 and JP 7268132, all of which are hereby incorporated by reference. Although some of the core compositions described in these disclosures are satisfactory, a need remains for an improved composition for forming golf ball cores.
For example, U.S. Pat. No. 4,929,678 relates to a golf ball formed from a polybutadiene core composition having a broad Mooney viscosity of 45-90, preferably 50-70, and more preferably 55 to 65. However the dispersity of the core composition is limited to the range of 4.0 to 8.0, and preferably 4.0 to 6.0. According to the ""678 patent, a dispersity of less then 4.0 produces deleterious workability.
Similarly, U.S. Pat. No. 5,082,285 generally discloses the preparation of a solid golf ball from an ultra-high molecular weight polybutadiene having a number average molecular weight of 40xc3x97104 or more, which has dispersity characteristics as noted. See also U.S. Pat. Nos. 4,974,852 and 5,585,440, wherein Mooney viscosity is discussed without reference to dispersity.
Accordingly, it is an object of the present invention to provide an improved polybutadiene composition which, when utilized to formulate golf ball cores, produces golf balls exhibiting enhanced C.O.R. without increasing hardness. An additional object of the invention is to produce a golf ball core from a polybutadiene composition having a high Mooney viscosity and/or a high molecular weight and low dispersity.
Accordingly, it is another object of the present invention to provide an improved core composition which, when utilized to formulate golf ball cores, produces golf balls exhibiting enhanced C.O.R. and improved processing.
The spin rate and xe2x80x9cfeelxe2x80x9d of a golf ball are particularly important aspects to consider when selecting a golf ball for play. A golf ball with the capacity to obtain a high rate of spin allows a skilled golfer the opportunity to maximize control over the ball. This is particularly beneficial when hitting a shot on an approach to the green.
Golfers have traditionally judged the softness of a ball by the sound of the ball as it is hit with a club. Soft golf balls tend to have a low frequency sound when struck with a club. This sound is associated with a soft feel and thus is desirable to a skilled golfer.
Balata covered wound golf balls are known for their soft feel and high spin rate potential. However, balata covered balls suffer from the drawback of low durability. Even in normal use, the balata covering can become cut and scuffed, making the ball unsuitable for further play. Furthermore, the coefficient of restitution of wound balls is reduced by low temperatures.
The problems associated with balata covered balls have resulted in the widespread use of durable ionomeric resins as golf ball covers. However, balls made with ionomer resin covers typically have PGA compression ratings in the range of 90-100. Those familiar with golf ball technology and manufacture will recognize that golf balls with PGA compression ratings in this range are considered to be somewhat harder than conventional balata covered balls. It would be useful to develop a golf ball having a durable cover which has the sound and feel of a balata covered wound ball.
These and other objects and features of the invention will be apparent from the following summary and description of the invention and from the claims.
The present invention achieves all of the foregoing objectives and provides, in a first aspect, a golf ball comprising a core that includes a particular combination of polybutadiene rubbers, and a cover disposed about the core which includes a specific combination of ionomer resins. The polybutadiene rubbers used in the particular combination include a first polybutadiene rubber that is obtained utilizing a cobalt catalyst and which exhibits a Mooney viscosity in the range of from about 70 to about 83. The combination of polybutadiene rubbers also includes a second polybutadiene rubber that is obtained utilizing a neodymium series catalyst and which exhibits a Mooney viscosity of from about 30 to about 70. The cover composition used in this golf ball includes a combination of three ionomers. That combination includes a sodium ionomer, a magnesium ionomer, and a zinc ionomer.
In a second aspect, the present invention provides a golf ball comprising a core that includes a particular combination of first and second polybutadiene rubbers, in particular weight proportions, and a cover generally enclosing the core which is formed from a certain combination of ionomers. Specifically, the core includes from about 30% by weight of a first polybutadiene rubber obtained from a cobalt or cobalt-based catalyst, and about 30% to about 45% by weight of a second polybutadiene rubber obtained from a neodymium or lanthanide-series catalyst. The cover of the golf ball includes a sodium ionomer, a magnesium ionomer, and a zinc ionomer.
In yet another aspect, the present invention provides a golf ball comprising a core including a certain combination of polybutadiene rubbers, which is enclosed with a cover formed from a particular set of ionomers. The core includes from about 20% to about 30% by weight of a first polybutadiene rubber obtained from a cobalt catalyst and which exhibits a Mooney viscosity in the range of from about 70 to about 83. The core further includes from about 30% to about 45% of a second polybutadiene rubber obtained from a neodymium catalyst and which exhibits a Mooney viscosity of from about 30 to about 70. The cover which is disposed about the core includes from about 40% to about 50% of a sodium ionomer, less than 45% of a magnesium ionomer, and from about 5% to about 15% of a zinc ionomer.
In a further aspect, the present invention comprises a blend of metal ion ionomers which include acidic copolymers. In this preferred embodiment of the present invention, the golf ball cover includes three ionomers, a magnesium ionomer, a sodium ionomer, and a zinc ionomer. Preferably, the magnesium ionomer consists of about 41 weight percent of the blended cover. The sodium ionomer preferably consists of about 47 weight percent of the cover and the zinc ionomer preferably consists of about 12 or 13 weight percent of the blended ionomer cover.
Further, it is preferred that the sodium ionomer include a low acid copolymer, preferably a 15% acrylic acid. Further, the magnesium ionomer includes a high acid copolymer, preferably a 19% methacrylic acid. The third ionomer in the present invention, the zinc ionomer, also preferably includes a low acid polymer, for example, a 15% acrylic acid.
Further scope of the applicability of the invention will become apparent from the detailed description provided below.